1. Field of Invention
This invention applies to testing of semiconductor memories and in particular testing auto-refresh and self refresh circuitry for Synchronous DRAM's.
2. Description of Related Art
During an auto-refresh for a SDRAM (Synchronous DRAM) an address from the internal counter is decoded to turn on a word line. All cells along the word line are refreshed After the refreshing is complete the word line is automatically turned off and the internal counter is incremented by one. The same process happens in a self refresh operation, except the self refresh operation is executed every sixteen microseconds. It is very difficult to test auto-refresh or self refresh for a SDRAM because the refresh operations are self timed. During refresh the word line address is also difficult to read to know which word line is being refreshed.
Involved in refreshing a SDRAM are the internal counter, auto-refresh control logic and self refresh control logic. If the counter is incremented by one after either an auto-refresh or a self refresh, then it has been held that the refresh circuitry worked properly.
In U.S. Pat. No. 4,672,583 (Nakaizumi) a DRAM (dynamic random access memory) is equipped with a test circuit for testing the internal refresh circuitry. An internal address counter is used to supply an address to both the row and column address decoders such that one memory cell is designated along a diagonal in the memory cell array. Each designated memory cell is written into and then read to insure the correct data was written. Referring to U.S. Pat. No. 5,321,661 (Iwakiri et al.), test circuitry is included on a DRAM to allow testing of the on chip refresh circuitry. In U.S. Pat. No. 5,625,597 (Hirose) test circuitry is included on a DRAM to test the refresh counter and to the counter cycle at time of refresh.
It is the purpose of this invention to present a method to test auto-refresh circuitry and a similar method to test self refresh circuitry in an easy and concise way for an SDRAM. It is further a purpose of this invention to be able to accurately determine on which word line a refresh failure occurred.